1. Field of the Invention
The present invention relates to a system that moves in water filling up a hollow structure such as a nuclear reactor and performs inspection or repair of a member disposed in the water.
2. Description of the Related Art
Core internals, or structures in a pressure vessel, of a nuclear reactor, specifically a boiling-water reactor (BWR), are made of a material enduring high-temperature and high-pressure conditions and having high-temperature strength, such as an austenitic stainless steel or a nickel-base alloy.
The material forming the core internals are subjected to harsh environment and neutron irradiation, and thus deteriorate after long time use. Specifically, since the material near welded parts of the core internals is sensitized by welding heat and has tensile residual stress, there exists a risk of stress corrosion cracking. A shroud, which is one of main structures in a reactor vessel and is hardly exchange, has the above risk. Thus, the shroud, specifically welded parts thereof, must be inspected periodically and repaired according to the inspection result.
There have been proposed various systems for performing inspection and/or repair of a reactor vessel or core internals. Such a system is provided with a working tool, and is configured to freely move in waters or is configured to move in water while being flattened against a wall. In detail, a known system has a main body provided with an inspecting and/or repairing tool (hereinafter referred to as “work tool”). The main body is provided therein with a float so that the main body may be suspended in water in its vertical posture. The main body is further provided with a thrust fan and drive wheels. By operating the thrust fan, the main body approaches a vertical wall, and the drive wheels come into contact with the wall. Then, the drive wheels are driven to run on the wall, so that work tool attached to the main body of the system moves along the wall surface to reach a location to be inspected or repaired. Such a system is disclosed in Japanese Patent Laid-Open Publication JP09-58586A and JP2003-40194A, for example.
In the foregoing system whose main body is suspended in water in its vertical posture, the vertical position of the main body may possibly be dislocated due to its own weight or force acting on the main body through a cable connected thereto. Since the direction of the dislocation is perpendicular to the running direction of the main body (i.e., horizontal direction), it is difficult to correct the vertical position of the main body to its target position. That is, the work tool can not be moved along a path to be inspected and/or repaired.